Nervous System Structure and Function: Study Notes for Anatomy & Physiology

Functions of the Nervous System

Overview

The nervous system is the master control and communication system in the body, responsible for every thought, action, and emotion. It uses sensory receptors to monitor changes, processes information, and initiates responses.

• Sensory Input: Uses sensory receptors to monitor changes (stimuli) inside and outside the body.

• Integration: Processes and interprets sensory input to decide what actions are necessary.

• Motor Output: Activates muscles or glands to respond to stimuli.

Nervous system functions are similar to a feedback loop, where information is received, processed, and results in motor response.

Organization of the Nervous System

Structural Classification

• Central Nervous System (CNS): Comprises the brain and spinal cord; acts as the command center, interpreting sensory information and issuing instructions.

• Peripheral Nervous System (PNS): Includes all nerves outside the CNS; consists of spinal nerves (spinal cord) and cranial nerves (brain); serves as communication lines between the body and the CNS.

Functional Classification

• Sensory Division (Afferent): Carries impulses from sensory receptors to the CNS; keeps it informed of external and internal changes.

• Motor Division (Efferent): Transmits impulses from the CNS to muscles and glands; causes motor response.

  • Somatic Nervous System: Controls voluntary movements of skeletal muscles.

  • Autonomic Nervous System (ANS): Manages involuntary functions, with subdivisions:

    • Sympathetic: "Fight or Flight"

    • Parasympathetic: "Rest and Digest"

Neuroglia Cells of the Nervous System

Central Nervous System (CNS) Neuroglia

• Astrocytes: Support neurons, maintain environment, and remove debris.

• Ependymal Cells: Line brain and spinal cord cavities, produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Wrap around CNS nerve fibers, forming myelin sheaths that insulate and increase speed of nerve transmission.

Peripheral Nervous System (PNS) Neuroglia

• Schwann Cells: Form myelin sheaths around PNS nerve fibers; essential for insulating nerve fibers and speeding transmission.

• Satellite Cells: Cushion and protect neuron cell bodies in the PNS; regulate chemical environment.

Myelin Sheath

Structure and Function

The myelin sheath is a whitish, fatty material that covers most long nerve fibers and has a waxy appearance.

• Protection and Insulation: Insulates and protects nerve fibers, preventing electrical impulses from leaking out.

• Increased Speed of Transmission: Myelin increases the speed at which nerve impulses travel.

• Structure and Formation: In the PNS, Schwann cells create myelin sheaths by wrapping around the axon in a jelly-roll fashion. In the CNS, oligodendrocytes form the sheath.

• Nodes of Ranvier: Gaps in myelin sheath crucial for rapid transmission of nerve impulses.

Multiple Sclerosis (MS)

Cause and Effects

Multiple Sclerosis is an autoimmune disease where the immune system attacks the myelin sheaths in the CNS.

• Formation of hardened areas (scleroses) disrupts the normal flow of electrical impulses.

• Results in nerve signals not reaching their intended targets, causing symptoms such as visual/speech disturbances, muscle control loss, and increased disability.

Exact cause of MS is not fully understood; it involves the immune system attacking a protein component of the myelin sheath.

Key Terms in Nervous System Anatomy

• Ganglia: Small collections of neuron cell bodies outside CNS and inside PNS; serve as relay stations.

• Nuclei: Clusters of neuron cell bodies within the CNS; carry out essential metabolic functions.

• Tracts: Bundles of nerve fibers in the CNS; transmit signals within the CNS.

• Nerves: Bundles of nerve fibers in the PNS; serve as communication lines.

• White Matter: Dense collections of myelinated nerve fibers (tracts); found in the CNS.

• Grey Matter: Mostly unmyelinated fibers and neuron cell bodies; found in regions such as cerebral cortex and basal nuclei.

Functional Classification of Nerves

Sensory Division (Afferent)

• Somatic Sensory Fibers: Deliver impulses from skin, skeletal muscles, and joints.

• Visceral Sensory Fibers: Transmit impulses from visceral organs.

Motor Division (Efferent)

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System: Regulates involuntary activities such as smooth and cardiac muscle and glands.

  • Sympathetic: "Fight or Flight"

  • Parasympathetic: "Rest and Digest"

Structural Classification of Neurons

• Multipolar Neurons: Most common type; many processes extending from the cell body; include motor and association neurons.

• Bipolar Neurons: Rare; found in special sense organs (e.g., eyes, nose); two processes (one axon, one dendrite).

• Unipolar Neurons: Typically found in sensory neurons within the PNS; single process divides into two branches.

Nerve Cell Functional Properties

• Irritability: Ability of neurons to respond to a stimulus by generating nerve impulses.

• Conductivity: Ability to transmit impulses to other neurons, muscles, or glands.

Irritability leads to Action Potential/Nerve Impulse

Action Potential Generation and Propagation

Steps in Action Potential

1. Resting Membrane Potential: The inside of the neuron is negatively charged compared to the outside due to the distribution of ions (mainly Na+ and K+).

2. Stimulus Initiates Local Depolarization: A stimulus changes the membrane permeability, allowing Na+ ions to enter the cell, making the inside more positive.

3. Depolarization and Generation of Action Potential: If the stimulus is strong enough, depolarization reaches a threshold, and an action potential is generated.

4. Propagation of Action Potential: The action potential travels along the axon as more Na+ channels open sequentially.

5. Repolarization: K+ ions diffuse out of the cell, restoring the negative charge inside.

6. Initial Ionic Conditions Restored: The Na+-K+ pump restores original ion concentrations.

Key Equation:

$\text{Resting Membrane Potential} \approx -70\ \text{mV}$

$\text{Na}^+/\text{K}^+\ \text{Pump:} \ 3\ \text{Na}^+\ \text{out},\ 2\ \text{K}^+\ \text{in}$

All-or-Nothing Response

Action potentials either occur fully or not at all. If the threshold is reached, the neuron fires; if not, no action potential is generated.

HTML Table: Comparison of CNS and PNS Neuroglia

Neuroglia Type	Location	Main Function
Astrocytes	CNS	Support neurons, maintain environment, remove debris
Ependymal Cells	CNS	Produce and circulate cerebrospinal fluid (CSF)
Oligodendrocytes	CNS	Form myelin sheaths, insulate nerve fibers
Schwann Cells	PNS	Form myelin sheaths, insulate nerve fibers
Satellite Cells	PNS	Cushion and protect neuron cell bodies

HTML Table: Structural Classification of Neurons

Neuron Type	Number of Processes	Location	Function
Multipolar	Many	CNS	Motor and association neurons
Bipolar	Two	Special sense organs	Sensory processing
Unipolar	One (divides into two branches)	PNS	Sensory neurons

Example: Application of Action Potential

When you touch a hot surface, sensory receptors in your skin generate an action potential that travels via sensory neurons to the CNS, where the information is processed and a motor response is sent to withdraw your hand.

Additional info: Some explanations and terminology have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.